The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Internal combustion engines may be operated at a lean air-to-fuel (A/F) ratio to improve fuel economy. Nitrogen oxide (NOx) emissions produced during lean operation are controlled. Selective catalytic reduction (SCR) catalysts, dosing systems, and lean NOx trap (LNT) catalysts are commonly used with internal combustion engines for emissions reduction.
In a typical SCR process, NOx reacts with a reductant which is injected by the dosing system into the exhaust gas stream to be absorbed onto an SCR catalyst. The injected dosing agent (e.g., urea) breaks down to form ammonia (NH3). NH3 reacts with NOx to reduce NOx into nitrogen (N2) and water (H2O).
LNT catalysts may absorb NOx from exhaust gas when the SCR unit cannot effectively reduce NOx emission during an engine start-up period. LNT catalysts may release the absorbed NOx after the exhaust gas reaches a predetermined temperature where the SCR unit can effectively convert NOx into N2 and H2O. As a result, NOx emission released to the atmosphere during the engine start-up period may be reduced.